1. Field of the Invention
The present invention relates to membrane separation processes and, more particularly, to composite-type membranes which may be used in such processes. The membranes include microporous supports which are coated with polymer compositions which have been cured with ultraviolet ("UV") light. The present invention also relates to a method for making such membranes.
2. Description of the Prior Art
Membrane separation processes are used widely in many fields of technology. These processes generally involve the permeation of gases or liquids through polymeric membranes where the driving force for their transport is either pressure or concentration. The membrane prevents hydrodynamic flow so that transport therethrough is by sorption or diffusion. Membranes are typically selective in what they transport and are considered semipermeable if, under identical conditions, they transport different molecular species at different rates. The rate of transport through membranes is a function of its permeability and is generally referred to as the flux.
Gas permeation processes generally involve the permeation of gases on the high-pressure side of the membrane to the low-pressure side. Liquid permeation, for example, involves the permeation of feed components from the liquid phase on one side of the membrane to the liquid or vapor phase on the other side.
As discussed above, the selectivity of separation membranes is an important factor in the satisfactory operation of membrane separation processes. In addition, membrane properties such as flux and resistance to chemical, biological and physical degradation also affect the efficiency of separation processes.
Of course, there have been many efforts to develop membranes which function efficiently in separation processes. Typical of such efforts include the development of composite-type membranes such as those disclosed in U.S. Pat. Nos. 4,242,159, 4,260,652, 4,277,344 and 4,388,189. These membranes include a microporous support having coated thereon a thin layer of polymeric material. However, previously known composite membranes have not been completely satisfactory since they can or may exhibit a variety of defects which affect physical, chemical and biological degradation resistance and flux, and thus the overall efficiency of the membrane processes for which they are used.
In an effort to overcome the above described deficiencies in composite-type membranes there have recently been attempts to produce composite-type membranes wherein the polymeric coating material is a UV curable material. The basic UV curable formulation generally includes a UV reactive unsaturated polymeric material, a photocatalyst and a reactive diluent.
For example, Japanese Kokai Patent No. Sho 59-76504 (1984) describes a reverse osmosis membrane which is manufactured by impregnating a porous support with a mixture of monofunctional monomer and bifunctional monomer and irradiating the mixture with light to polymerize the monomers. Furthermore, U.S. Pat. No. 4,613,533 also suggests that membranes coated with polymeric material may be cured using ultraviolet light.
As a general matter, however, there has been very little work done in examining the potential use of these type of membranes in separation processes which operate on a solution/diffusion mechanism such as gas separation, reverse osmosis, pervaporation and liquid/liquid separation. One possible explanation for the lack of development in this area is the failure of this type of membrane to exhibit a high flux value. That is, coating UV reactive mixtures onto porous supports followed by curing has been found to result in membranes having low flux. The conventional wisdom is that the UV reactive mixtures, after coating, tend to "wick up" and fill the pores of the microporous support thereby producing a membrane having an insufficient flux and unsuitable effective thickness which includes the porous support. This failure is unfortunate since UV curable systems have the potential to be tremendously advantageous in the area of membrane separation processes since a wide range of chemical and mechanical properties may be built into the polymeric systems thereby producing membranes having improved resistance to physical, chemical and biological degradation. Also, the simplicity of these systems compared to conventional systems is potentially appealing in that they involve solventless processes and curing by UV light. Curing by UV light is also advantageous since it is less expensive on a commercial scale than methods such as heat which have been used for curing conventional polymeric systems. The development of a useful UV curable resin coated composite membrane for application in membrane separation processes would therefore be an extremely important and desirable development.